Hi Andrea, I have a two-year old son, too. Don'tcha just love 'em??! I have so much fun with him...but he caught on to a bad word a few months back, too. Sadly, I didn't get to blame it on anyone but DH (darling Hubby). Got him to stop after a couple weeks, though (son, not the hubby).
Ignore your son the next time he says the word...if he's insistent that you pay attention to him, then completely misunderstand him (e.g. "Frog? What frog, honey?") He'll try a few more times when it occurs to him, but if you continue with the above (no yelling and especially, no laughing) then the fun will be over before you know it. Good luck!